To employ a plug to affect blockage of a normally open flow passage for repair purposes has been known in the prior art in connection with many types of fluid flow systems. Examples of expansible plugs or dams of the prior art may be found in U.S. Pat. Nos. 2,843,154; 3,834,422; and, 4,518,015. From these concepts, which are generally applicable to flow passageways, nozzle plugs for steam generators of conventional nuclear power generating systems have developed. An example of such a nozzle plug for effecting a temporary blockage of a passage into or out of a nuclear power system steam generator will be found in U.S. Pat. No. 4,482,076 to Timothy H. Wentzell, assigned to Combustion Engineering, Inc., the assignee of the instant invention.
Periodically, there arises a need to conduct maintenance on, and to effect minor repairs of, the internal components of the steam generator. To accomplish such tasks, it is necessary for one or more persons to physically enter the steam generator. Ingress and egress to and from the steam generator by such persons is accomplished through suitable means such as, for example, an access port or manway. While such persons are working in the steam generator, it is desirable that a blockage of the inlet and outlet nozzle of the steam generator be effected to ensure that there will be no fluid flow through the steam generator which might physically imperil the people working therein.
One such design of a nozzle plug for a nuclear steam generator is disclosed in the U.S. Pat. No. 4,482,076 mentioned above. In that design, a plurality of plate sections are assembled within the steam generator to form the plug or nozzle dam. U.S. Pat. No. 4,483,457 to Schukei and Tade discloses an improvement over the Wentzell patent in that it provides for the sections of the nozzle dam to be hinged together.
Both of these patents teach dams which rely upon locking pins which enter inserts in holes in the side wall of the nozzle to hold the dams in place. The pins are tethered on lanyards and loosely attached to the dam sections until the dam is placed in its approximate final position in the nozzle. They are then manually inserted through holes in bosses on the dam structure and into the inserts. The process of finally aligning the dam boss holes, insert holes and tethered pins, usually is difficult and requires two men. Invariably the lanyards tangle, slowing the operation. Two independent nozzle dams are usually used in each steam generator.